Method of producing highly emissive electrodes



Patented Oct. 10, 1950 METHOD OF PRODUCING HIGHLY EIVHSSIVE ELECTRODES Michel E. Macksoud, Flushing, Y."

No Drawing. Application August 25, 1950,

' Serial No. 181,567

My invention relates to the preparation of an electrode for use in gaseous arc discharge tubes, particularly mercury vapor lamps.

The most important object of my invention is to produce a more efficient electron emissive electrode.

Another object of the invention is to eliminate electrode sputtering and tube blackening in arc discharge devices, as well as to increase the useful life thereof. I

An important feature of the invention resides in a method of preparing an electrode including first forming barium azide crystals upon the surface and in the convolutions and interstices of a coil of thorium or zirconium and thereafter treating the coated coil to free the barium and form at the surface of the coil an alloy consisting essentially of barium and either thorium or zirconium.

The present application is a continuation-inpart of my oopending application Serial No. 9,183 filed February 18, 1948 and entitled Method of Producing Highly Emissive Electrodes.

In my copending application I have disclosed the process of preparing an emissive electrode in which barium azide crystals are formed either upon a tungsten coil or upon a tungsten coil upon which has been positioned thorium or zirconium. I have now found that an electrode of somewhat superior characteristics may be produced by eliminating the tungstenand by first coating a substantially pure thorium member (or zirconium), with barium azide crystals and thereafter treating the coated member to form an alloy com posed of barium and thorium (or zirconium).

An electrode thus formed appears superior to those heretofore known, in the case of thorium, because of mild radio activity. When a tube containing such an electrode is first energized most of the emission comes from the barium, but as the temperature rises, the thorium accounts for the greater part of the emission. I have found that such electrodes exhibit even less sputtering than those described in my copending application and also exhibit longer useful life. Moreover the ionizing potentials is somewhat lower,

thus providing a tube more easily energized andv therefore more fiexible in use. In the case of zirconium, the electrode is particularly effective because of the voracious gettering characteristic of this metal.

In carrying out the process of my invention I first provide thorium or zirconium in suitable forms, that is to say, the thorium or zirconium may be in the form of a cable composed of small 5 Claims. (01. 1l7-33.3)

diameter strands of the metal twisted in cable formation and then coiled to form a helix. Alternatively a relatively large diameter wire of the metal may be provided in coiled coil form. An electrode may also be formed of a twisted mesh of thorium or zirconium wires. To obtain the most efficient form of such an electrode it is extremely important that the thorium Or zirconium coil and all leads thereto be thoroughly cleaned. One of several satisfactory cleaning agents is hydrogen peroxide. If this agent is used the coil is boiled for a suitable period in a concentrated solution of-hydrogen peroxide and then fired in hydrogen at approximately 1350 C. for about five minutes. Other suitable cleansing techniques are available and understood by those skilled in the art. Then the cleaned coil is carefully mounted on a beaded lead wire either by welding or clamping. Then the mounted coil is dipped into a barium azide solution and subsequently baked in a dry low-temperature oven at approximately 200 C. for approximately five minutes. This entire process may be repeated a second time in order to develop a heavier coating of barium azide crystals upon the electrode. It is extremely important that this first heating operation be carried out under such circumstances as to avoid the production of oxides. If temperatures higher than 210 C. are used, the heating time should be correspondingly cut down, similarly lower temperatures may be used provided the heating time is increased. The important thing, as I have stated above, is to form barium azide crystals as a coating upon the coil of thorium or zirconium without the production of oxides. If the barium is oxidized the finished electrode will sputter and blacken the tube.

' Moreover barium oxide Will not form the alloy which is essential to the product of the process of my invention.

The barium azide solution may be in the form of a supersaturated solution having approximately fifty per cent distilled water as a suspension medium or suitable portions of alcohol and/ or ether may be added to increase the rapidity of the evaporation rate of the solution. Without water the crystals formed by this barium azide solution are monoclinic. However, when in suspension with Water and alcohol the crystals are triclinic, the index of refraction is 1.7. The solution is designated as Ba(N3) 21-120. After the electrodes have been dipped in barium azide solution and subsequently baked in the lowtemperature oven, it will be observed that a densely packed layer of triclinic crystals is formed on the 3 electrode surface, this coating extending into and filling the crevices and interstices of the electrode coil assembly.

In sealing the electrode to the glass tube envelope, a forming gas or suitable inert gas such as nitrogen or argon must be employed to prevent oxidation of the electrode. It will be noted that when heating the glass envelope with the sealing fire burners just prior to sealing,

small sparks will be thrown off from the crevices of the glass envelope tube and the beaded electrode assembly. This is due to the initial decomposition of the azide. During the processing of the tube while being exhausted to a high degree of vacuum, the electrodes must be further activated either by high frequency bombardment or by high current arcing. The preferred method of activation is that of high frequency bombardment which results in an electrode of higher uniformity in electron emissive properties, and the glass envelope tube has .less tendency to blacken due to any sputtering :aotion. During this processing the azide is completely broken down, releasing considerable nitrogen and leaving a pure metallic barium on the surface of the electrode.

Although the matter has not as yet been determined beyond doubt, all the indications point to the formation of an alloy of barium and thorium or zirconium in the form of a surface coating upon the main body of the electrode. I have observed that the barium does not vaporize from the coil when the arc is established, although the temperature at the electrode is much greater than that at which barium will vaporize. Moreover the emissivity does not decrease significantly, an effect almost certain to result if the barium left the coil. Also the striking potential remains much lower than could be accounted for if the barium left the coil. For these reasons I feel quite certain that some form of alloy is produced. The vital point, and the essence of my invention, resides in my discovery that the beneficial results described here cannot be accomplished if barium azide is treated in the fashion conventional in the art. That is to say, the formation of the crystals of the barium azide upon the coil will produce a stable, highly emissive electrode. If barium azide is applied in other forms, such as a carbonate, the electrode will not exhibit the fine qualities obtainable by my process. An electrode conventionally coated with barium azide or barium carbonate will sputter, lose emissivity, and blacken the arc tube and reduce they a output therefrom.

The are tube during the process of high vacuum exhaust is first baked to a high temperature to out-gas the glass walls of the envelope and simultaneously to affect an initial clean-up of the occluded gases in the electrodes of the tube. During this process considerable nitrogen is released from the azide crystals formed on the electrodes to initiate the breaking down of these crystals into pure metallic barium. Further activation of these electrodes may be carried out by inductively heating the electrodes with high frequency currents, or forming a relatively high current are discharge between the electrodes for a predetermined period of time. Two separate high frequency bombardments or two separate arcing cycles, each of which may be for not more than fifteen seconds, are sufficient to preactivate the electrodes prior to final filling with a predetermined pressure of inert 4 gas such as argon and a predetermined quantity of triple distilled mercury.

After the high vacuum exhaust, cathode activation and processing, and final filling, the tube is now ready for the seasoning process. This consists of placing the mercury tube in a heat shielded chamber and passing a current between the electrodes approximately equivalent to the current used in normal operation. This process is continued for approximately one half hour during which the operating characteristics of the arc tube become stabilized and uniform, the gas content of the tube appears to be cleaned up and impurities dispelled, together with the formation of an integral unitary electrode that is highly electron emissive.

To summarize. The process of my invention comprises essentially the steps of first forming barium azide crystals as a coating upon an electrode of thorium or zirconium and thereafter heating the coated assembly at an elevated temperature,'either by high frequency bombardment or by subjecting the member to an electric current of high amperage, for such length of time as is required to disassociate the barium azide to form metallic barium upon the electrode, and to alloy the barium with either the zirconium or the thorium.

Having thus disclosed my invention what I claim as new and desire to secure by Letters Patent of the United States is:

l. The method of preparing highly emissive electrodes, which includes the steps of applying to a member selected from the group consisting of thorium and zirconium an aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subsequently heating the coated member at an elevated temperature in the substantial absence of oxygen and for a sufficient length of time to disassociate the barium azide toform metallic barium on the member and alloy said barium with the member.

2. The method of preparing highly emissive electrodes, which includes the steps of applying to a thorium member an aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subsequently heating th coated member at an elevated temperature in the substantial absence of oxygen and for a sufficient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the thorium member.

3. The method of preparing highly emissive electrodes, which includes the steps of applying to a zirconium member an aqueous solution of barium azide, heating the coating member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subsequently heating the coated member at an elevated temperature in the substantial absence of oxygen and for a sumcient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the zirconium member.

4. The method of preparing highly emissive electrodes which includes the steps of applying to a thorium member and aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subjecting the coated member to high frequency bombardment in the substantial absence of oxygen and for a suflicient length of time to disassociate the barium azide to form metallic barium on the member and alloy said barium with the member. I

5. The method of preparing highly emissive electrodes which includes the steps of applying to a zirconium member an aqueous solution of barium azide, heating the coated member at a non-oxidizing temperature of about 200 C. and until the barium azide is crystallized out and deposited on the surface of the member, and subjecting the coated member to high frequency bombardment in the substantial absence of oxygen and for a suificient length of time to disassociate the barium azide to form metallic barium on the member and. alloy said barium with the member.

MICHEL E. MACKSOUD.

REFERENCES CITED The following references are of record in the file of this patent: 

1. THE METHOD OF PREPARING HIGHLY EMISSIVE ELECTRODES, WHICH INCLUDES THE STEPS OF APPLYING TO A MEMBER SELECTED FROM THE GROUP CONSISTING OF THORIUM ANDD ZICRONIUM AN AQUEOUS SOLUTION OF BARIUM AZIDE, HEATING THE COATED MEMBER AT A NON-OXIDIZING TEMPERATURE OF ABOUT 200*C. AND UNTIL THE BARIUM AZIDE IS CRYSTALLIZED OUT AND DEPOSITED ON THE SURFACCE OF THE MEMBER, AND SUBSEQUENTLY HEATING THE COATED MEMBER AT AN ELEVATED TEMPERATURE IN THE SUBSTANTIAL ABSENCE OF OXYGEN AND FOR A SUFFICIENT LENGTH OF TIME TO DISASSOCIATE THE BARIUM AZIDE TO FORM METALLIC BARIUM ON THE MEMBER AND ALLOY SAID BARIUM WITH THE MEMBER. 